1. Field of the Invention
The present invention relates to a printing apparatus to detect multi-feeding of sheets transferred along a sheet transfer route.
2. Description of the Related Art
A printing apparatus circulates a plurality of sheets separately by feeding one by one from a stack of sheets in a sheet feed rack of a feeding mechanism. Meanwhile, the plurality of sheets may be overlapped and transferred together when passing through a printing mechanism. This is so-called “multi-feeding of sheets”. Japanese Patent Laid-Open Publication No. 2001-063872 discloses a technology for detecting multi-feeding of sheets by means of a light transmission sensor as a multi-feed detection sensor provided on a sheet transfer route. The light transmission sensor measures the amount of light transmission of a sheet on the sheet transfer route so as to detect the multi-feeding of sheets since the amount of light transmission is dependent on a thickness of a sheet.
FIG. 1A shows one example of conventional arrangements of light transmission sensors used for detecting multi-feeding of sheets. In this figure, a sheet from a sheet feeding side is transferred to register rollers 750 to pause for positioning a front edge part of the sheet. Then, the sheet is guided to print heads 712 by the register rollers 750.
In the sheet transfer route, a light transmission sensor 760 composed of a light emitter 762 and a light-receiving sensor 764 is provided on the sheet feeding side of the register rollers 750. While, a light transmission sensor 770 composed of a light emitter 772 and a light-receiving sensor 774 is provided on a sheet exit side of the register rollers 750. The light transmission sensor 760 serves as a register sensor to detect a sheet entering the register rollers 750. The light transmission sensor 770 serves as an edge part detection sensor to detect edge parts of the sheet further transferred to the print heads 712.
The light transmission sensor 760 provided on the sheet feeding side of the register rollers 750 also serves as a multi-feed detection sensor in order to detect multi-feeding of sheets on the feeding side as quickly as possible.